1. Field of the Invention
The present invention relates to an optical scanner reflecting/deflecting a light beam such as a laser beam for scanning an object.
2. Description of the Background Art
In general, a two-dimensional image apparatus such as a laser printer or a scanner is mounted with an optical scanner precisely scanning an object with a laser beam. This type of optical scanner reflects/deflects the laser beam with a light deflector such as a galvanometer mirror or a polygon mirror for scanning an objective surface of a photosensitive drum or the like. While the light deflector rotates at an equiangular velocity, the laser beam must scan the objective surface at a uniform rate. Therefore, the optical scanner employs an f-xcex8 (ef-theta) lens as an optical system letting the laser beam reflected/deflected by the light deflector scan the objective surface at a uniform rate. The f-xcex8 lens is an optical system having a distortion characteristic satisfying y=fxcfx89(f: focal distance, xcfx89: half angle of view) in relation to an ideal image height y.
FIGS. 11 and 12 show a conventional optical scanner mounted with an f-xcex8 lens 104. FIG. 11 is a schematic block diagram of the optical scanner developed along a Y-Z plane, and FIG. 12 is a longitudinal sectional view developing the optical scanner shown in FIG. 11 along an optical axis. Referring to FIGS. 11 and 12, numeral 100 denotes a light source (semiconductor laser), numeral 101 denotes a collimator lens, numeral 102 denotes a cylindrical lens, numeral 103 denotes a polygon mirror, numeral 104 denotes the f-xcex8 lens, numeral 105 denotes an anamorphic lens and numeral 106 denotes an objective surface. Directions X, Y and Z shown in FIGS. 11 and 12 are perpendicular to each other.
The light source 100 oscillates a laser beam 107 directly modulated by a driving circuit (not shown). This laser beam 107 is parallelized by the collimator lens 101 and converged by the cylindrical lens 102 for forming a linear image on a reflecting surface 103r of the polygon mirror 103. The polygon mirror 103 rotates about a rotational axis 103c by tens of thousands of revolutions per minute and the f-xcex8 lens 104 is an optical system converting equiangular velocity motion of incident light from the reflecting surface 103r to uniform motion, whereby a light beam reflected by the reflecting surface 103r of the polygon mirror 103 is deflected at an equilateral velocity and scans the objective surface 106 in the direction Y. The anamorphic lens 105 converges light incident from the f-xcex8 lens 104 perpendicularly (direction X) to a primary scanning direction (direction Y) for forming an image on the objective surface 106.
As shown in FIG. 11, the light beam scans the objective surface 106 over a scanning line length W, and hence the f-xcex8 lens 104 must have a wide total angle xcex8 of view. Further, the size of an image has recently been so increased that an optical scanner having a large scanning line length W is required. Assuming that f represents the focal distance of the f-xcex8 lens 104 at the working wavelength for the light beam, the following relational expression holds:
W=fxcex8
When the scanning line length W is enlarged while keeping the total angle xcex8 of view constant, therefore, the focal distance f of the f-xcex8 lens 104 is increased. In order to enlarge the scanning line length W while keeping the focal distance f of the f-xcex8 lens 104 constant, on the other hand, the total angle xcex8 of view must be increased. In this case, the aperture of the f-xcex8 lens 104 is so increased that it is difficult to precisely work the f-xcex8 lens 104 and correct optical aberration values thereof, to readily increase the cost for the f-xcex8 lens 104.
Compactification of the optical scanner has also been required in recent years. As shown in FIG. 13, an f-xcex8 lens 104 built in the optical scanner is formed by three groups of lenses, i.e., a first lens 111 having negative refracting power, a second lens 112 having positive refracting power an a third lens 113 having positive refracting power. Between the total length L (face-to-face distance between an entrance-side curved surface 111i of the first lens 111 and an exit-side curved surface 113e of the third lens 113) of the f-xcex8) lens 104 and a focal distance f, the following relational expression holds:
0.100xe2x89xa6L/fxe2x89xa60.108
Hence, the total length L exceeds 0.100xc3x97f. An f-xcex8 lens having optical performance not deteriorated also when the total length L is further reduced has recently been required.
The present invention is directed to an optical scanner reflecting/deflecting a light beam such as a laser beam for scanning an object.
According to the present invention, the optical scanner comprises a light deflector periodically reflecting a light beam emitted from a light source to periodically deflect said light beam and an imaging optical system having such a distortion characteristic that the product of a focal distance and a half angle of view defines an ideal image height for imaging the light beam deflected by the light deflector on an objective surface, and the imaging optical system comprises a first lens having negative refracting power, a second lens having positive refracting power and a third lens having positive refracting power successively from an entrance side for the light beam to satisfy the following expressions (1) and (2):                               L          f                 less than         0.100                            (        1        )                                0.10        ≤                  r1          r3                ≤        0.26                            (        2        )            
where L represents the length between a plane of incidence of the first lens and a plane of exit of the third lens along an optical axis direction and f represents the composite focal distance of the first lens, the second lens and the third lens in the above expression (1) while r1 represents the radius of curvature of a refracting interface on the entrance side for the light beam in the first lens and r3 represents the radius of curvature of a refracting interface on the entrance side for the light beam in the second lens in the above expression (2).
A compact imaging optical system can be formed with a total length L smaller as compared with a focal distance f by satisfying the above expression (1), thereby implementing a compact optical scanner. Further, the imaging optical system can properly correct bending of a meridional image surface by satisfying the above expression (2). According to the present invention, both conditions of the above expressions (1) and (2) are compatible with each other, whereby a compact optical scanner having high optical performance can be manufactured.
Preferably, the first lens, the second lens and the third lens are made of an optical material satisfying the following expression (4) on the basis of a partial Abbe""s number xcexd defined in the following expression (3):                     υ        =                                            N              A                        -            1                                              N              MIN                        -                          N              MAX                                                          (        3        )                                1.40        ≤                              υ                          p              ⁢                              xe2x80x83                            ⁢              s                                            υ                          n              ⁢                              xe2x80x83                            ⁢              g                                      ≤        1.70                            (        4        )            
where NA represents a refractive index with respect to the central wavelength of a working wave range of the light beam, NMIN represents a refractive index with respect to the lower limit of the working wave range of the light beam and NMAX represents a refractive index with respect to the upper limit of the working wave range of the light beam in the above expression (3) while xcexdps represents the partial Abbe""s number of the second lens and the third lens and xcexdng represents the partial Abbe""s number of the first lens in the above expression (4).
An imaging optical system capable of correcting on-axis chromatic aberration within tolerance can be implemented by satisfying the above expression (4).
More preferably, the imaging optical system satisfies the following expression (5):                     0.26        ≤                              "LeftBracketingBar"            f1            "RightBracketingBar"                    f                ≤        0.33                            (        5        )            
where f1 represents the focal distance of the first lens in the above expression (5).
An imaging optical system capable of further properly correcting bending of a meridional image surface can be implemented by satisfying the above expression (5).
More preferably, the imaging optical system is formed to satisfy the following expression (6):                     0.41        ≤                  f3          f                ≤        0.66                            (        6        )            
where f3 represents the focal distance of the third lens in the above expression (6).
An imaging optical system capable of improving a scanning property of a light beam can be implemented by satisfying the above expression (6).
More preferably, another imaging optical system converging the light beam emitted from the light source only in the direction of the rotational axis of the light deflector and imaging the light beam on a reflecting surface of the light deflector is further provided on an optical path between the light source and the light deflector, and the imaging optical system further comprises an anamorphic lens converging a light beam outgoing from the third lens in the direction of the rotational axis of the light deflector and imaging the light beam on the objective surface.
When displacement is present in perpendicularity of the reflecting surface of the light deflector, inclination of reflected light resulting from this displacement can be so corrected that the light beam can precisely scan the objective surface at a regular pitch.
When employing a light beam having a central wavelength of around 405 nm, the imaging optical system more preferably satisfies the following expression (2A):                     0.10        ≤                  r1          r3                ≤        0.26                            (                  2          ⁢          A                )            
A compact imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 405 nm.
When employing the light beam having the central wavelength of around 405 nm, the optical material more preferably satisfies the following expression (4A) with respect to the light beam having the central wavelength of around 405 nm:                     1.44        ≤                              υ                          p              ⁢                              xe2x80x83                            ⁢              s                                            υ                          n              ⁢                              xe2x80x83                            ⁢              g                                      ≤        1.70                            (                  4          ⁢          A                )            
An imaging optical system capable of correcting on-axis chromatic aberration within a proper range can be implemented particularly with respect to the light beam having the central wavelength of around 405 nm.
When employing the light beam having the central wavelength of around 405 nm, the imaging optical system more preferably satisfies the following expression (5A) with respect to the light beam having the central wavelength of around 405 nm:                     0.28        ≤                              "LeftBracketingBar"            f1            "RightBracketingBar"                    f                ≤        0.33                            (                  5          ⁢          A                )            
An imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 405 nm.
When employing a light beam having a central wavelength of around 635 nm, the imaging optical system more preferably satisfies the following expression (2B):                     0.11        ≤                  r1          r3                ≤        0.25                            (                  2          ⁢          B                )            
A compact imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 635 nm.
When employing the light beam having the central wavelength of around 635 nm, the optical material more preferably satisfies the following expression (4B) with respect to the light beam having the central wavelength of around 635 nm:                     1.50        ≤                              υ                          p              ⁢                              xe2x80x83                            ⁢              s                                            υ                          n              ⁢                              xe2x80x83                            ⁢              g                                      ≤        1.62                            (                  4          ⁢          B                )            
An imaging optical system capable of correcting on-axis chromatic aberration within a proper range can be implemented particularly with respect to the light beam having the central wavelength of around 635 nm.
When employing the light beam having the central wavelength of around 635 nm, the imaging optical system more preferably satisfies the following expression (5B) with respect to the light beam having the central wavelength of around 635 nm:                     0.30        ≤                              "LeftBracketingBar"            f1            "RightBracketingBar"                    f                ≤        0.33                            (                  5          ⁢          B                )            
An imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 635 nm.
When employing a light beam having a central wavelength of around 785 nm, the imaging optical system more preferably satisfies the following expression (2C):                     0.12        ≤                  r1          r3                ≤        0.21                            (                  2          ⁢          C                )            
A compact imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 785 nm.
When employing the light beam having the central wavelength of around 785 nm, the optical material more preferably satisfies the following expression (4C) with respect to the light beam having the central wavelength of around 785 nm:                     1.40        ≤                              υ                          p              ⁢                              xe2x80x83                            ⁢              s                                            υ                          n              ⁢                              xe2x80x83                            ⁢              g                                      ≤        1.70                            (                  4          ⁢          C                )            
An imaging optical system capable of correcting on-axis chromatic aberration within a proper range can be implemented particularly with respect to the light beam having the central wavelength of around 785 nm.
When employing the light beam having the central wavelength of around 785 nm, the imaging optical system more preferably satisfies the following expression (5C) with respect to the light beam having the central wavelength of around 785 nm:                     0.26        ≤                              "LeftBracketingBar"            f1            "RightBracketingBar"                    f                ≤        0.31                            (                  5          ⁢          C                )            
An imaging optical system capable of properly correcting bending of a meridional image surface can be implemented particularly with respect to the light beam having the central wavelength of around 785 nm.
Accordingly, an object of the present invention is to provide a compact optical scanner comprising an f-xcex8 lens having a small total length L and high optical performance also when a focal distance f as well as a scanning line length W are increased.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.